This invention relates to reducing solid particle erosion (hereinafter SPE) in axial flow fluid turbines, such as steam turbines, and more particularly, to reducing erosion of the trailing, or downstream, edge of stationary spaced apart nozzle partitions, which are used to define passageways, or nozzles, therebetween for directing steam flow into a rotatable plurality of turbine blades.
In general, steam turbines operate to convert energy stored in high pressure, high temperature steam, such as may be obtained from an external boiler, into rotational mechanical movement. Steam turbines employed by electric utilities as a prime mover for electrical generators to produce electric power, typically comprise a plurality of turbine blades, or buckets, radially extending and circumferentially mounted on the periphery of a rotor shaft to form a turbine wheel. Generally, the steam turbine includes a plurality of axially spaced apart bucket wheels. The rotor shaft, with associated bucket wheels, is mounted on bearings with the bucket wheels disposed inside an inner shell which may be in turn surrounded by a spaced apart outer shell. This double shell configuration forms a pressurizable housing in which bucket wheels rotate and prevents potentially damaging thermal gradients.
The bucket wheels are disposed between corresponding stationary nozzle diaphragms which are formed by an array of stationary aerodynamically configured partitions substantially radially disposed between and fixedly retained by a pair of concentric diaphragm rings, which circumferentially surround the rotor. These partitions are typically referred to as nozzle partitions and the spaces between the partitions as nozzles. As steam flows through the interior cavity of the pressurizable inner shell, it passes through and coacts with alternately disposed stationary nozzle partitions and rotatable turbine bucket wheels to produce rotational movement of the rotor shaft. The combination of a pair of diaphragm rings with their associated partitions and the cooperating row of downstream buckets is generally referred to as a stage, stages being numbered sequentially in the direction of steam flow starting from the steam input region. These concepts are elementary and are generally well known in the turbine art.
Modern large steam turbines generally comprise several sections such as, for example, high-pressure (HP), intermediate pressure (IP) and low-pressure (LP), which may be mechanically coupled to drive an electrical generator. Of course other turbine configurations are possible, such as a double reheat turbine which includes a high-pressure, first reheat (IP), second reheat ("low pressure" IP) and low pressure sections. Generally the reheat portion of a turbine is defined to include all intermediate and low pressure sections, i.e. from the outlet of a steam reheater coupled between the high-pressure and first intermediate-pressure section to the input of a condenser for condensing steam before recycling the water formed back to the steam generator. These sections possess various design characteristics so as to permit extraction of the optimum amount of energy from the expansion of steam through the respective turbine sections, thereby optimizing overall turbine efficiency. It is common practice to have one or more of these sections configured in a double flow arrangement, in which steam entering a middle portion, or tub, of the section encounters a diverging flow path. After entry into this middle portion of one of the turbine sections, steam exits in substantially opposite directions, wherein the oppositely directed steam flows are used to impart rotation in the same direction to the turbine shaft. This double flow configuration beneficially contributes to overall machine efficiency. However, the present invention is applicable to all generally axial flow fluid turbines, regardless of steam flow path diversions.
In a system configured so that steam flows from a boiler to a high pressure turbine and then successively to an intermediate pressure and low pressure turbine, it has been noted that the trailing edge portion of nozzle partitions, especially those of the first stage of the reheat portion, are subject to SPE. SPE is believed to be caused by exfoliation of an oxide film, which is primarily magnetite (Fe.sub.3 O.sub.4), from the steam side of boiler tubes and steam conduit. Until detailed investigation by applicants identified the actual mechanism of SPE of trailing edges of nozzle partitions in the first stage of the reheat section of the turbine, it was believed that SPE was caused by direct impact on the pressure side of nozzle partitions by contaminating particles entrained in the steam flow. However, applicants surprisingly have discovered that the velocity of particles entrained in the steam flow through nozzles was relatively constant, and not substantially affected by the rapidly increasing steam velocity as it approached the throat region (i.e. minimum flow area between adjacent nozzle partitions) of the nozzle. Further, the vast majority of particles were found to be impacting the trailing edge region of the pressure surface of a nozzle partition at a relatively steep angle in contrast to relatively shallow angles that are required to produce maximum rates of SPE. In addition, it was noted that particles impacting the pressure side lost momentum and therefore approached buckets at an angle and velocity that are conducive to producing rebound, after collision with buckets, back upstream against the generally axial flow of steam to strike the suction surface of partitions.
Continuous operation of a turbine in an environment conducive to SPE may eventually result in loss of metal along the trailing edge portion of nozzle partitions, whereby the designed airfoil configuration of partitions is altered. This results in reduced stage, and thereby overall turbine, efficiency. SPE may also be a factor contributing to forced outages, extended maintenance outages, shortened planned inspection intervals and increased maintenance costs, all of which adversely affect economical operation of the turbine.
In order to remove the suspected source of the SPE problem for turbines, work has progressed on reducing undesirable material supplied with steam to the turbine from boilers. However, it is still desirable to find a solution to the problem of SPE at the turbine, since it is expensive to replace entire tubes or rework the steam side of boiler tubes of existing boilers, and even with such modifications, boilers may still experience some exfoliation.
Using sophisticated computer modelling techniques to model steam flow through turbine sections, applicants have discovered an unexpected mechanism of SPE which is particularly functional in the first stage of the reheat section and other stages where the steam pressure level is relatively low as compared with the input steam pressure level to the first stage of the high pressure section. They have found that a major component of the mechanism for SPE of trailing edges of nozzle partitions, especially for the first stage of a reheat section of a turbine, includes particles exiting nozzle passages of the first stage and striking the leading edge portion of the associated downstream rotating buckets of the first stage. After striking the buckets, the particles rebound off the buckets and strike the suction surface of first stage nozzle partitions with sufficient velocity and at an appropriate angle to cause SPE of the nozzle partitions. Thus, contrary to prior belief, the primary cause of SPE of the trailing edge of reheat first stage nozzle partitions has been determined by applicants to be particulate impingement on the suction side of the trailing edge portion of nozzle partitions from a direction opposing steam flow through the turbine stage.
Accordingly, it is an object of the present invention to prevent solid particle erosion of the trailing edge portion of nozzle partitions by particles impinging the trailing edge portion on the suction side of nozzle partitions.
Another object of the present invention is to prevent solid particle erosion of the endwall of a diaphragm ring.